Magnetic sensors are an important aspect in emerging technologies. A small, lightweight, hysteresis-free and maintenance-free magnetic sensor is key to many applications such as automotive sensors, memory modules, computer hard disk storage devices, and M-RAMs.
Magnetic sensors common today employ a variety of sensor structures. One such structure is the single member Anisotropic Magnetoresisitive (“AMR”) sensor such as the structures described in U.S. Pat. Nos. 4,503,394; 4,663,684; and 4,425,593 herein fully incorporated by reference. AMR sensors are typically permalloy thin-film deposited on a silicon wafer, or other type of substrate, and patterned as elongated rectangular strips. The permalloy is generally deposited in a strong magnetic field that sets the preferred orientation of the magnetization vector in the sensor. The resistance of the strips varies with respect to an angle formed between a sensed magnetic field vector and the electrical current vector within the sensor. The strip resistance is maximized when the direction of the sensed magnetic field is parallel to the direction of the electrical current within the sensor. In order to have a more hysteresis-free response, the sensor must not change its magnetic alignment. Therefore, the sensor is generally kept in a particular magnetization alignment. If the sensor is exposed to a disturbing magnetic field, which will break down the magnetization alignment in the sensor, a strong magnetic field must be applied along the length of the thin-film to restore the sensor. Large external magnets, or smaller individual coils are often placed adjacent to the sensor to maintain the sensor's preferred orientation. Alternatively, current straps, also known as set-reset straps, may be used to restore the sensor's characteristics. The use of current straps in a magnetic field sensing device is discussed in U.S. Pat. No. 5,247,278 to Bharat B. Pant, hereby fully incorporated by reference.
Multilayered magnetic sensors typically utilize the giant magnetoresistive (“GMR”) effect. The GMR effect is observed in magnetic multilayer structures, where a thin rectangular spacer layer about one nm (nanometer) thick separates two rectangular, magnetically-soft, ferromagnetic layers. The GMR effect is analogous to a pair of polarizers where aligned polarizers allow light to pass through, but crossed polarizers do not. The first ferromagnetic layer allows electrons in only one spin state to pass through easily. Likewise, the second ferromagnetic layer allows electrons in only one spin state to pass through easily. Therefore, if the first ferromagnetic layer is aligned to allow the same spin state as the second ferromagnetic layer then the electrons can easily pass through the structure, and the resistance is low. If the second ferromagnetic layer is misaligned to the first ferromagnetic layer then neither spin state can get through the structure easily and the electrical resistance is high.
The GMR effect basically measures the difference in angle between the two magnetizations in the ferromagnetic layers. Small angles (parallel alignment of electron spin) give a low resistance, and large angles (antiparallel alignment of electron spin) give a high resistance. To produce the state where the two ferromagnetic layers are parallel one applies a field large enough to magnetically saturate both layers. However, controllable rotation of the magnetization over a wide range of magnetic fields is quite difficult to achieve. The magnetization vectors rotate continuously up to some critical angle at which an irreversible transition into a collinear state occurs. Since the value of this angle is typically small, the corresponding changes in the electrical resistance due to the GMR effect are also small. Consequently, applications of such device for sensing magnetic fields would be limited due to the bipolar nature of the response (no field or in-field). For this reason magnetically-soft, ferromagnetic sensors utilizing the GMR effect are typically employed in applications that require measurements of relatively small magnetic fields. Magnetically-soft, ferromagnetic sensors utilizing the GMR effect include unpinned sandwich, antiferromagnetic multilayer, and spin valve structures.
An unpinned sandwich sensor 1, shown in FIG. 1, typically includes a first layer 5, a second layer 10, and a third layer 15. The first layer 5 is made of a magnetically-soft, ferromagnetic material. The second 10 layer is made of a nonmagnetic, electrically conductive material and typically about 3-5 nm thick. The third layer 15 is made of a magnetically-soft, ferromagnetic material. The second layer 10 is connected to and on-top of the first layer 5. The third 15 layer is connected to and on-top of the second layer 10. Permalloy is a common magnetically-soft, ferromagnetic material used in the first layer 5 and the third layer 15. Typically the second layer 10 is made of copper. An unpinned sandwich structure is usually patterned into narrow elongated rectangular strips. In order to have a more hysteresis-free response, the sensor must be maintained so the first layer 5 and the third layer 15 are magnetically antiparallel. The sensor must be reset by a magnetic field rotating the first layer 5 and the third layer 15 into antiparallel or high resistance alignment. A positive or negative external field parallel to the sensor has the same change in resistivity. An unpinned sandwich structure generally has up to a 5% change in resistivity across the sensor and is saturated by a 2.4 to 5 kA/m (30-60 Oe) magnetic field.
An antiferromagnetic sensor is exactly the same as the unpinned sandwich sensor 1, shown in FIG. 1, except the second layer is thin enough to cause antiferromagnetic coupling between the first layer 5 and the third layer 15. The second layer 10 is about 1.5-2 nm thick, for a copper spacer. This antiferromagnetic coupling causes the first layer 5 to have a magnetic moment antiparallel to the third layer 15. These antiparallel magnetic moments are required to keep the sensor operating in a high resistance state, allowing for maximum sensitivity to an external magnetic field 17. In order to have a more hysteresis-free response, these antiparallel magnetic moments must be maintained. An external magnetic field 17, if large enough, can overcome the coupling which causes this alignment and can align the magnetic moments so that all the layers are parallel, causing a low resistance state. Once this occurs the sensor must be reset by the application and subsequent removal of an external magnetic field 17 having a strong magnetic field along the length of the sensor 1. An antiferromagnetic multilayer structure generally results in a change of up to a 14% change in resistivity across the sensor.
A spin valve sensor is also similar to the unpinned sandwich sensor except a fourth layer, an antiferromagnetic layer is added. A spin valve sensor 50, shown in FIG. 2a, typically includes a first 55, second 60, third 65, and fourth 70 layer. The second layer 60 is connected to and on-top of the first layer 55. The third 65 layer is connected to and on-top of the second layer 60. The fourth layer 70 is connected to and on-top of the third layer 65. The first layer 55 is made of an antiferromagnetic layer and pins the adjacent second layer 60 into a particular direction of electron spin. The second layer 60 is made of a magnetically-soft, ferromagnetic material. The third layer 65 is made of an electrically-conducting/non-magnetic material. The fourth layer 70 is made of a magnetically-soft, ferromagnetic material. The direction of the first layer 55, the pinning layer, is usually fixed by raising the temperature of the sensor above the blocking temperature. Above the blocking temperature the second layer 60 is no longer magnetically coupled to the adjacent anti-magnetically-soft, ferromagnetic first layer 55. The sensor 50 is then cooled in an external magnetic field 17 strong enough to fix the direction of the magnetic moment in the first layer 55 (pinned layer). Since the change in magnetization in the free layer is due to rotation rather than magnetic domain wall motion, hysteresis is reduced, but still present. A spin valve sensor generally results in a change between 4 to 20% change in resistivity between the first layer 55 and the fourth layer 70. A spin valve sensor generally becomes saturated at fields 0.8 to 6 kA/m (10 to 80 Oe).
Ferromagnetic/nonmagnetic/metal-granular alloys (typically Co—Cu) are a group of materials which exhibit the GMR effect with only a single layer. At room temperature, the solubility of Co in Cu (Ag) is practically negligible, however, special preparation methods allow the production of a metastable solid solution. After thermal treatment, a more stable situation is achieved by the precipitation of small Co clusters in the Cu (Ag) matrix, thus they are called granular alloys. Granular alloys can be alloys of magnetically-soft, ferromagnetic Ni, Co, Fe with nonmagnetic Ag, Au, Cu, etc. Developing theories propose that the GMR effect arises from the spin-dependent scattering that takes place mainly at the interface. The great advantage of granular alloys sensors compared to multilayered sensors is that they are much simpler to prepare. Granular alloys do not require accurate control of the thickness of the various layers during the growth process. Granular alloys can be deposited at room temperature. However, applications of granular materials as candidates for magnetic field sensing are limited by a relatively lower GMR effect (only a few percent) and resulting low sensitivity. Furthermore, granular-alloys are also plagued by hysteresis.
Another magnetic sensor effect is the tunnel magnetoresistive (TMR) effect. Structures that have the TMR effect have the same general form as structures having the GMR effect, but the interlayer non-magnetic material is electrically insulating rather than electrically conducting. In a structure utilizing the TMR effect, a voltage is applied between magnetically-soft, ferromagnetic films cause a tunneling current to flow across the interlayer with a magnitude that depends on the relative orientation of the magnetization on both sides of the interlayer. This results in a larger change in resistivity than similar GMR effect systems. As with GMR effect, the resistance is higher for anti-alignment.
An example of a magnetic sensor using the TMR effect is the spin dependent TMR effect sensor 80, shown in FIG. 2b including a first 85, second 90, third 95, and fourth 100 layer. The second layer 90 is connected to and on-top of the first layer 85. The third layer 95 is connected to and on-top of the second layer 90. The fourth layer 100 is connected to and on-top of the third layer 95. The first layer 85 is made of an antiferromagnetic layer and pins the adjacent second layer 90 into a particular direction of electron spin. The second layer 90 is made of a magnetically-soft, ferromagnetic material. The third layer 95 is made of an electrically-insulating/non-magnetic material. The fourth layer 100 is made of a magnetically-soft, ferromagnetic material. Since the change in magnetization in the fourth layer 100 is due to rotation rather than magnetic domain wall motion, hysteresis is reduced. A spin dependent TMR effect sensor 80 structure generally results in a 10 to 25% change in resistivity between the first layer 85 and the fourth layer 100. A typical spin dependent TMR effect sensor 80 becomes saturated at fields 0.1 to 10 kA/m (1 to 100 Oe).
All of these structures suffer from hysteresis. When an external magnetic field is applied to these ferromagnetic structures, the energy is stored. As a result, the magnetization state of the magnetic material depends not only on the magnitude of an applied field, but also on the field history of the material. This causes a nonlinear relationship between the magnetic field strength (H) and the magnetic flux density (B) within such materials. If the relationship between the magnetic field strength (H) and the magnetic flux density (B) is plotted for increasing levels of field strength, it will follow a curve up until a point where further increases in the magnetic filed strength (H) will result in no further change in flux density (magnetic saturation). If the magnetic field is then reversed and increased linearly, the relationship will again follow a similar curve back towards and beyond zero flux density (B), but offset from the original curve by the remanence (absence of magnetic field). This is caused by the tendency of the ferromagnetic material to retain some of the magnetic flux density (B), which must be overcome each time the magnetic field across the ferromagnetic material is reversed.
Optimally, one would like to fabricate a sensor that is self-resetting (does not require a antiferromagnetic layer or a reset-strap) and has a hysteresis-free, tunable response.